DESCRIPTION (from applicant's abstract): Toxoplasma gondii is an obligate intracellular parasite, which is capable of invading and replicating within all nucleated cells. Toxoplasmosis, particularly involving the brain, is a major source of morbidity and mortality in the Acquired Immune Deficiency Syndrome. The vacuole membrane surrounding the intracellular parasite contains a functional pore, which allows diffusion of small nutrients from the host cell into the vacuolar space surrounding the parasite. However, essentially nothing is known about macromolecules of host cell origin required for parasite growth, nor of mechanisms for parasite acquisition of these nutrients. We have demonstrated that intracellular T. gondii tachyzoites efficiently acquire and sequester host cell cholesterol internalized into cells through the host cell LDL receptor pathway. Viable parasites are required for this process, potentially implicating a secreted parasite sterol binding protein. Providing excess cholesterol to infected cells accelerates T. gondii growth, suggesting that cholesterol is limiting for parasite growth. T. gondii induces an increase in LDL internalization in infected host cells, possibly by activating host cell regulatory mechanisms of cholesterol homeostasis. These are all completely unexpected findings for a parasite, which resides in a vacuole, which does not fuse with organelles of the host endocytic cascade. To pursue the above observations, the mechanism by which the parasite regulates expression of host genes involved in sterol uptake and biosynthesis will be determined. Secreted parasite proteins, which participate in cholesterol acquisition from the host cell will be identified. Endocytic traffic in the parasite will be perturbed using pharmacologic and molecular manipulations, and effects on cholesterol uptake and delivery to parasite organelles will be assessed. These experiments are important because they elucidate the mechanisms of transport of an essential nutrient cholesterol - sequentially from the host cell to the PVM and PV, then across the parasite plasma membrane an ultimately to parasite organelles. Simultaneously, they define the contribution of the T. gondii endocytic pathway to macromolecule uptake, and open the possibility that this pathway can be used for therapeutic advantage in T. gondii and potentially in other Apicomplexan parasites, most notably Cryptosporidium parvum.